CHAPTER IV

THE CHARACTERISTICS OF THE ZONES: I.-—THE

TROPICS

General: Climate and Weather-Temperature-The SeasonsPhysiological Effects of Heat and Humidity-Pressure-Winds and Rainfall-Land and Sea Breezes-Thunderstorms-Cloudiness-Intensity of Skylight and Twilight-Climatic Subdivisions:-I. The Equatorial Belt.-II. Trade Wind Belts.-III. Monsoon Belts.-IV. Mountain Climate.

General: Climate and Weather. The so-called "torrid zone has been variously bounded. Its limits have been set at the tropics (lat. 2312°); at the mean annual isotherms of 68°, which also correspond closely with the poleward extension of palms; and at the polar margins of the trade winds. The dominant characteristic of this great belt, embracing but a little less than one-half of the earth's surface, is the remarkable simplicity and uniformity of its climatic features. This simplicity is reflected in the striking regularity in the recurrence of the ordinary weather phenomena. The tropics lack the proverbial uncertainty and changeableness which characterise the weather of the higher latitudes. In the torrid zone, weather and climate are essentially synonymous terms. Periodic phenomena, depending upon the daily and annual march of the sun, are dominant.. Non-periodic weather changes are wholly subordi

nate. The succession of daily weather changes is even more regular, and the distribution of the climatic elements is even more uniform over the tropical oceans than over the lands. In special regions only, and at special seasons, is the regular sequence of weather temporarily interrupted by an occasional tropical cyclone. These cyclones, although comparatively infrequent, are notable features of the climate of the areas in which they occur. Generally bringing very heavy rains, and thus locally increasing the total annual precipitation by a considerable amount, they yet cause no marked temperature changes such as those which are the common accompaniments of extratropical cyclones. The devastation produced by one of these storms often affects the economic condition of the people in the district of its occurrence for many

years.

Temperature. The sun is always well up in the sky. The length of day and night varies little. Hence the mean temperature is high, it is very uniform over the whole zone, and there is little variation during the year. The mean annual isotherm of 68°. is a rational limit at the polar margins of the zone, and the mean annual isotherm of 80° encloses the greater portion of the land areas, as well as much of the tropical oceans. The isotherms are thus far apart. The warmest latitude circle for the year is not the equator, but latitude 10° north. The highest mean annual temperatures, shown by the isotherm of 85°, are in central Africa, in India, the north of

Australia and Central America, but, with the exception of the first, these areas are small. Massowah, on the Red Sea, has an annual mean of over 86°. The temperatures average highest where there is little rain, and not in the belt of heavy equatorial rains, where the clouds afford some protection from the sun's rays. In June, July, and August there are large districts in the south of Asia, and in northern Africa, with temperatures over 90°. Winds blowing out from these heated deserts are uncomfortably hot and dusty.

Over nearly all of the zone the mean range of temperature is less than 10°, and over much of it, especially the oceans, it is less than 5°. At Equatorville, in the interior of Africa, on the Congo, the mean annual range is only a little over 2°; at Iquitos (lat. 3.7° S.), in Peru, it is 4.3°. Even near the margins of the zone, where the seasonal differences are greatest, the ranges are less than 25°, as at Calcutta, Hong Kong, Rio de Janeiro and Khartum. The mean daily range is usually larger than the mean annual. Thus at Equatorville the former is about 14.5°. It has been well said that "night is the winter of the tropics." The differences between the maximum and minimum temperatures of the year near the equator are not much greater than the daily range. Over an area covering parts of the Pacific and Indian Oceans, from Arabia to the Caroline Islands and from Zanzibar to New Guinea, as well as on the Guiana coast,

the minimum temperatures do not normally fall below 68°, and over much of the torrid zone as a whole they do not fall below 59°. Towards the margins of the zone, however, the minima on the continents fall to, or even below, 32°. Maxima of 115°, and even over 120° (122°), occur over the deserts of northern Africa. A district where the mean maxima exceed 113° extends from the western Sahara to northwestern India, and over central Australia. Near the equator the maxima are therefore not as high as those

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in many so-called temperate climates. The

greater portion of the torrid zone is a water surface, and marine conditions are therefore typical for most of it. These tropical oceans show remarkably small variations in temperature. The Challenger results showed a daily range of hardly 0.7° in the surface water temperature on the equator, and Schott determined the annual range as 4.1° on the equator; 4.3° at latitude 10°, and 6.5° at latitude 20°. It has been clearly pointed out by Hann that the uniform distribution of temperature throughout the year-the dominant feature of the tropics-results not only from (1) the small variation in insolation and in the length of the day; but also (2) from the great extent of the zone, which makes it impossible for cold winds from higher latitudes to penetrate into the lower latitudes; (3) the oblique course of the trades, which are well warmed on their indirect road towards the equator; (4) the slight nocturnal cooling, where the air

is damp and vapour is readily condensed; and (5) the great extent of the tropical oceans, which gives so much of the zone a marine climate.

The Seasons. In a true tropical climate, seasons, in the temperate zone sense, do not exist. The variations in temperature throughout the year are so slight that the seasons are not classified according to temperature, but depend on rainfall and the prevailing winds. The life of animals and plants in the tropics, and of man himself, is regulated very largely, in some cases almost wholly, by rainfall. Agriculture prospers, or fails, according to the sufficiency and punctual appearance of the rains. After a long dry season, when the rains come, there is an extraordinarily sudden awakening of the parched and dusty vegetation. Where, on the other hand, there is abundant moisture throughout the year, a tree may at the same time be carrying buds, blossoms, and ripe fruit. Vegetation under these conditions has been well called non-periodic. Although the tropical rainy season is characteristically associated with a vertical sun (i. e., summer), that season is not necessarily the hottest time of the year. The temperature is usually somewhat lower under the clouds. The rainy season often goes by the name of winter for this reason, and also because the weather is dull. The time of the maximum temperature is also controlled by the rainy seaTowards the margins of the zone, with increasing annual ranges of temperature, seasons in the extra-tropical sense gradually appear.

son.